1. Field of the Invention
The present invention relates to means and method for providing large area hybrid electronic packaging on a single substrate.
2. Description of the Prior Art
Current hybrid technology generally consists of fabricating and assembling passive and active components in a single small enclosure which is usually made hermetic. Examples of such enclosures are ceramic and metal packages which are 1/4" square or 1" square having 14 or 30 leads. Thick and thin film techniques are utilized for producing passive circuit elements on substrates mounted within these packages, and uncased discrete devices are used for semiconductors and other circuit elements. The hybrid microcircuit thus produced is typically the electronic equivalent of an assembled printed wire board. The hybrid is ultimately mounted onto a printed wiring board along with other hybrids and discrete components. Accordingly, the hybrid may be said to represent the first level of integration and the assembled printed wiring board as the second level of integration. In such hybrid technology utilizing assembled printed wiring boards, most thick film hybrid work is restricted to substrates smaller than two inches square with very few multilayer techniques extending beyond two or three conductor levels.
Examples of some prior art techniques are described in the following publications: "Low Cost Packaging of Thick-Film Substrates With Ceramic Glass Seals" by J. C. Gioia, NEPCON PROCEEDINGS, 1970, Pages 10-42 to 10-46; "Aspects of Multilayered Thick-Film Hybrids" by R. G. Loasby, Solid State Technology, May 1971, Pages 33-37, 46; "Evaluation Testing of a Thick Film Multilayer Interconnect System" by M. W. Rossman, INTERNATIONAL HYBRID MICROELECTRONICS SYMPOSIUM, 1970, pages 6.3.1-6.3.9; "Fabrication of Multilayer Thick Film Microelectronic Circuits" by L. K. Keys, A. J. Russo, F. J. Francis and S. Herring, Jr. in INTERNATIONAL HYBRID MICROELECTRONIC SYMPOSIUM, 1970, Pages 6.4.1-6.4.10; and "Development of Large Thick-Film Multilayer Assemblies" by H. R. Isaak, J. W. Kanz and E. G. Babiracki in 1971 INTERNATIONAL MICROELECTRONICS SYMPOSIUM, Oct. 11-13, 1971, pages 1-27. Selected U.S. patents describing the prior art include U.S. Pat. Nos. 3,264,402; 3,302,067; 3,546,776; 3,581,375; 3,646,399; 3,674,914; and 3,691,628.
Prior art hermetically sealed packages fail to meet the requirements of a large area hybrid from several aspects. Four specific examples of these are discussed below. U.S. Pat. No. 3,609,294 speaks of thin film sputtering and etching, rather than thick film screened and fired circuits. It does not have a selective area hermetic seal nor is the disclosed device a large area structure. U.S. Pat. No. 3,602,634 speaks of a simple small semiconductor package, not a hybrid microcircuit with selective area seals. The cover is sealed by brazing rather than soldering. The insulating layers are of glass with deposited thin film aluminum conductors rather than a thick ceramic dielectric layer with thick film gold or platinum-gold screened and fired conductors. U.S. Pat. No. 3,753,054 discusses a large hermetic package designed to accommodate an LSI wafer, not a hybrid microcircuit. Hermeticity is obtained by brazing and welding rather than soldering and thus is not repairable. U.S. Pat. No. 3,673,309 discusses a single chip enclosure, rather than a hybrid microcircuit. The cover is brazed (not soft soldered) directly to the substrate rather than to an intermediate ring frame. In addition, it seals to a gold paste, which experiences leaching and scavenging problems, rather than to a platinum-gold alloy paste.
In general, the majority of prior art suffer from one basic respect. They are generally applicable only for small packages (usually less than one-inch on a side), and not to large structures--as large as 6-inches on a side. Processes and materials which work on small structures cannot simply be applied when the structure is expanded in area. In part, the largest problem to overcome is one of finding the proper combination of materials and processes which would work on large area structures while still maintaining the necessary reliability and yield.
The above-described prior art further suffers from one or more problems. The conventional printed wiring board design precludes the most effective use of board area and height required by the attachment of hybrid packages and other electronic components to the printed wiring board either directly or by wire bonds. Such a layout further results in extra weight which, for airborne and other uses, may result in a severe penalty in terms of space and added fuel consumption. The need to mechanically and electrically couple all individual components to the printed wiring board results in a large number of interconnections with increased possibility for resulting open and short circuits. In addition, because of the requirement to make a large number of interconnections, the complexity of each printed wiring board is limited. As a further problem, many components must be thermally coupled to a heat sink because of their high power loads. In general, conventional hybrid packages are mounted on a thermal supporting structure which, in turn, must be thermally coupled to a thermal dissipation means, that is, a heat sink. Because of the number of interfaces due primarily to the various adhesives used for bonding purposes in microcircuit fabrication, high thermal impedance occurs with the result that the requirement to cool the printed wiring board electronic system presents a limitation as to how much power can be achieved with each system.
Repair of such printed wiring board systems is relatively complicated. For example, if an individual hybrid package exhibits some fault, the package leads must be unsoldered and the package removed from the printed wiring board for replacement or repair. In the event of repair, the conventional package lid must be removed by heat applied to the sealing areas. Such heat may deleteriously affect otherwise functioning components within the hybrid package, thereby leading to further waste of material and components, in addition to loss and excessive use of manpower.